High bandwidth, reconfigurable interconnection networks have been heretofore suggested and/or utilized in digital electronic parallel processors and distributed computing networks. Such networks as have heretofore been utilized, however, are becoming communication limited as processing speeds continue to increase.
Optical interconnection techniques using fiber optics or free space broadcasting offer extremely high bandwidth capabilities, but routing flexibility and performance of currently available approaches are insufficient for many applications.
For example, heretofore known optical space division interconnection networks have included crossbars based on spatial masking and crossbars based on angular deflectors. These networks have utilized several different switching approaches including matrix-vector multipliers, beam deflection approaches based on acoustooptic devices, optical systolic and engagement arrays, holographic crossbars, optoelectronic photoconductor matrices, and networks of LiNbO.sub.3 directional couplers.
The first five approaches suffer from optical throughput inefficiencies due to fan-in and fan-out losses, although these limitations can in part be alleviated by using active deflectors and receivers. The directional coupler approach suffers coupling losses when trying to stitch together a large switch from arrays of small switches. Fan-out losses lead to an optical throughput efficiency for optical crossbars which fall off as 1/M, where M is the number of output nodes being interconnected. When the crossbar is used to interconnect an array of N single mode fibers at the input to an array of M single mode fibers at the output, an additional loss factor of 1/N is incurred due to fan-in losses, yielding an overall loss of 1/NM. For large networks this is an unacceptably low optical throughput, particularly in single mode fiber reconfigurable networks.
The optical matrix-vector multiplier based optical interconnection networks provide an array of N optical input data channels. The data channels, carried on fiber optic transmission lines, are spatially multiplexed in the x direction providing a uniformly spaced linear array. A sphere cylinder lens system images this array in the x direction and uniformly spreads and collimates the light in the y direction onto a 2-dimensional spatial light modulator (SLM). The SLM consists of an array of NxM pixels which can be selectively made opaque or transparent under electrical or optical control. A cylinder sphere lens system following the SLM collects all of the transmitted light from each row of the SLM and images the various columns onto an array of M output fibers which are uniformly spaced and multiplexed in y. For a crossbar interconnect, only one aperture is opened in each row and column. Thus, the SLM mask acts as a permutation matrix, redirecting light from each input fiber to the desired output fiber.
This system could implement a generalized crossbar with broadcast capabilities by opening more than one aperture in each column, and providing wired OR gates implemented by opening more than one pixel per row. However, optical losses suffered by this architecture (the fan-out of light from each input fiber to an entire column of M pixels with only one transmissive results in the 1/M fan-out losses, with fan-in losses being unavoidable due to the constant radiance theorem unless the output fiber has a larger mode volume than the input) makes this approach unattractive for many applications.
Several approaches to utilizing deflectors for interconnections have been advanced, including approaches based on magnetic domains, acoustooptic deflectors and holography. Such interconnections utilizing acoustooptics require one deflector per transmitter, with these stacked into a multichannel Bragg cell. An array of fiber inputs (N) are uniformly spaced and multiplexed in y, collimated in x and imaged in y onto a multichannel acoustooptic device, so that each acoustic channel is illuminated at the Bragg angle by one of the fiber inputs. This is accomplished with a grin rod lens array or a sphere-cylinder lens pair.
A plurality of piezoelectric acoustic transducers is driven by an RF single tone which deflects the diffracted light input from the fiber in the x direction by an amount proportional to the applied frequency. A Fourier transform lens is used to convert the diffraction angles into positions in x and to collect the light from each of the input channels onto an array of output fibers positioned along the locus of diffracted spots in the Fourier plane. Each output fiber corresponds to a single input RF frequency, and any input fiber can be deflected to a given output fiber by applying the appropriate input frequency. Thus, this system implements a crossbar with O(N) (O=order) acoustooptic devices.
While this system has substantially eliminated the fan-out losses when implemented in a permutation network, it still suffers fan-in losses when the mode volume of the output fibers is the same as that of the input fibers, resulting in a reduction of overall efficiency. In addition, while a broadcasting network can be implemented utilizing this approach by driving the transducer in the broadcasting channel with multiple RF frequencies, the diffraction efficiency into each receiving channel will fall off in proportion to the degree of fan-out.
Switching matrices made from arrays of directional couplers and intersecting waveguide switches have also been suggested for configurable optical interconnection applications. The directional coupler acts as a primitive 2.times.2 switch, and these can be combined into larger networks by using them as the N.sup.2 crosspoints of a crossbar or as the 3N log N exchange-bypass modules in a multistage interconnection network.
One of the main disadvantages of using these switches in a centralized switching facility is the long interaction length that is typically required, and the low bend radius that is a limitation of some waveguide technologies. In LiNbO.sub.3 directional couplers, the interaction length is on the order of millimeters and the allowed bend radius is so large that it would be difficult to fit more than about 9.times.9 crosspoints on a single substrate. The long range interconnections of a typical multistage interconnect network require sharp bends which are only conceivable when using etched rather than diffused guides.
While such LiNbO.sub.3 directional couplers are low loss (such systems having avoided fan-out and fan-in loss by using active devices, for example), when building larger systems by stitching together 9.times.9 switching matrices, the input-output coupling loss and propagation losses build up to as much as 25 dB for a 144.times.144 switching matrix. In addition, crosstalk builds up due to reflections and imperfect switching in such a matrix to such a large extent as to make them unusable for analog applications and to degrade the signal to noise ratio for digital applications as well.
As may be appreciated therefor, further improvement in such interconnection networks could be utilized which maintains high bandwidth and low latency characteristics in a readily reconfigurable interconnection network having low fan-in, fan-out and coupling losses.